Clarify docs on different tximport count files

Author: pmoris

docs/output.md

@@ -725,11 +725,11 @@ Transcripts with large inferential uncertainty won't be assigned the exact numbe
 
 The [tximport](https://bioconductor.org/packages/release/bioc/html/tximport.html) package is used in this pipeline to summarise the results generated by Salmon or Kallisto into matrices for use with downstream differential analysis packages. We use tximport with different options to summarize count and TPM quantifications at the gene- and transcript-level. Please see [#499](https://github.com/nf-core/rnaseq/issues/499) for discussion and links regarding which counts are suitable for different types of analysis.
 
-According to the `txtimport` documentation you can do one of the following:
+According to the [`txtimport` documentation](https://bioconductor.org/packages/release/bioc/vignettes/tximport/inst/doc/tximport.html#Downstream_DGE_in_Bioconductor) you can do one of the following:
 
-- Use bias corrected counts with an offset: import all the salmon files with `tximport` and then use `DESeq2` with `dds <- DESeqDataSetFromTximport(txi, sampleTable, ~condition)` to correct for changes to the average transcript length across samples.
-- Use bias corrected counts without an offset: load and use `salmon.merged.gene_counts_length_scaled.tsv` or `salmon.merged.gene_counts_scaled.tsv` directly as you would with a regular counts matrix.
-- Use bias uncorrected counts: load and use the `txi$counts` matrix (or `salmon.merged.gene_counts.tsv`) with `DESeq2`. This does not correct for potential differential isoform usage. Alternatively, if you have 3’ tagged RNA-seq data this is the most suitable method.
+- Use the original (bias-uncorrected) counts _with an offset_: import all the salmon `quant.sf` files with `tximport` and then use `DESeq2` with `dds <- DESeqDataSetFromTximport(txi, sampleTable, ~condition)` to automatically correct for changes to the average transcript length across samples by calculating a gene-level offset.
+- Use bias-corrected counts _without an offset_: load and use `salmon.merged.gene_counts_length_scaled.tsv` or `salmon.merged.gene_counts_scaled.tsv` directly as you would with a regular gene-level counts matrix. These files were created using the `tximport` argument `countsFromAbundance="scaledTPM"` or `"lengthScaledTPM"` to scale counts to library size or to library size and average transcript length respectively, forgoing the need for an offset matrix.
+- Use the original (bias-uncorrected) counts _without an offset_: load and use the `txi$counts` matrix (or `salmon.merged.gene_counts.tsv`) with `DESeq2`. This is generally **not** recommended, since it does not correct for potential differential isoform usage (the offset). However, if you have 3’ tagged RNA-seq data, then this _is_ the most suitable method, because the counts do not exhibit any length bias.
 
 > **NB:** The default Salmon parameters and a k-mer size of 31 are used to create the index. As [documented here](https://salmon.readthedocs.io/en/latest/salmon.html#preparing-transcriptome-indices-mapping-based-mode) and [discussed here](https://github.com/COMBINE-lab/salmon/issues/482#issuecomment-583799668), a k-mer size off 31 works well with reads that are 75bp or longer.